1. Field of the Invention
The present invention relates in general to a current control apparatus used in direct current (DC) motors and more particularly to a current control apparatus in which the driving state of the motor in battery driven electric vehicles is controlled by the current value.
2. Description of the Prior Art
Generally, battery driven apparatus such as electric forklifts, small electric cars, oil pressure device and swelling pumps use DC motors and DC motor controllers. These types of DC motors usually use a voltage control method.
FIG. 1 shows a pair of filtering condensers 2 and 3 for filtering the power from the battery 1, a motor field coil 4 and a motor armature 5 driven by the battery, a chopper transistor 6 for switching the battery power to motor field coil 4 and motor armature 5, a pair of forward and backward contactors 7 and 8 for controlling the polarity of the motor field coil 4, regeneration excitation circuit 9 for returning the energy generated by the motor system to the battery 1, regeneration excitation contactor 10 for switching the regeneration excitation circuit 9, and a microcomputer 11 for controlling the above.
Operations of the power circuit of DC motor will be described in detail hereinbelow.
Power from the battery 1 is filtered by the filtering condensers 2 and 3 and applied to the chopper transistor 6. The chopper transistor 6 performs switching operation of the battery power which is on-off controlled by the control signal of the microcomputer 11 to the motor field 4 and motor armature 5. The battery power is supplied to the motor field coil 4 and the motor armature 5 and then the motor begins to drive.
The battery 1 is charged by turning off the regeneration excitation circuit and regenerated when energy from the motor is supplied to the battery 1 through the regeneration diode 12. Thus the motor system loses kinetic energy and brakes.
However, there is a problem that the power circuit of DC motor is not efficiently controlled because the power circuit of the DC motor is made to control the input power required for driving the motor, the capacity of the filtering condenser on the input end is limited, and as a result, the input voltage cannot be increased above the predetermined value.
Also, there is another problem in that if the motor is used for a long time, the internal resistance of the motor will change depending on the temperature, and the inductance value in the motor will change, thereby causing an unstable current control.